fix(build): Cleanup dependency management

[matrei]

All dependencies were declared with the api scope which is not necessary and which leaks dependencies to downstream projects. After this change it should no longer be necessary for Grails apps to force the version of groovy-xml. Several unused dependencies have also been removed.

[matrei]

@puneetbehl I have now tested this pull request on https://github.com/grails/grails-functional-tests by:

• Publishing this branch as org.grails:grails-gradle-plugin:6.1.2-SNAPSHOT to mavenLocal
• Adding mavenLocal() as repository to all the subprojects buildscript sections in grails-functional-tests
• Changing to grailsGradlePluginVersion=6.1.2-SNAPSHOT (was 6.1.0) in gradle.properties in grails-functional-tests

And the build runs without issues. (The only thing I needed to change was geb/selenium/webdriver versions/config to be able to run the integration test suite it on my computer.)

I have also previously successfully built https://github.com/grails/grails-spring-security-core (which also has several grails apps for testing) with these changes. In this project there is also code in buildSrc/src/main/groovy which would not compile without forcing groovy-xml:3.0.13. With the changes in this branch the forcing can be removed.